Base station apparatus and reception band control method

ABSTRACT

A mobile station ( 14 ) transmits a signal after connection is established, by using the same number of subcarriers as the number of subcarriers equal to or smaller than a predetermined number used for transmitting a connection request signal (TCCH). A base station ( 12 ) detects the number of subcarriers used for transmitting the connection request signal (TCCH) (S 106 ), and controls in accordance with the detected number of subcarriers the passband width of a bandpass filter having a passband with a variable width accommodating the predetected number of subcarriers, for separating a signal of the mobile station ( 14 ) falling within the passband from a received signal (S 108 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. National Phase Application of International ApplicationNo. PCT/JP2009/053446 filed Feb. 25, 2009, which claims priority toJapanese Patent Application No. 2008-046536 filed Feb. 27, 2008, thedisclosures of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a base station apparatus and areception band control method, and in particular to a multicarriercommunication system using a plurality of subcarriers.

BACKGROUND ART

In a mobile communication system, in many cases, a transmission outputof a base station is generally larger than that of a mobile station, andan antenna height of the base station is higher than that of the mobilestation. Therefore, a range of a radio signal transmitted from themobile station to the base station (hereinafter, referred to as “uplinkbudget”) has a tendency to be smaller than a range of a radio signaltransmitted from the base station to the mobile station (hereinafter,referred to as “downlink budget”).

It should be noted that Patent Document 1 discloses a base stationapparatus for preventing reduction of frequency usage efficiency andincrease of power consumption by allocating fewer subcarriers to acontrol channel than to a traffic channel.

-   Patent Document 1: JP 3485860 B2

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

As described above, in a conventional mobile communication system, thereis a problem in that a communication area of the base station isnarrowed in a case where the uplink budget is smaller than the downlinkbudget.

The present invention has been conceived in view of the above describedproblem, and aims to provide a base station apparatus and a receptionband control method capable of extending a communication area of thebase station apparatus.

Means for Solving the Problems

In order to solve the above described problem, the present inventionprovides a base station apparatus for performing radio communicationwith a mobile station apparatus for transmitting a signal afterconnection is established, by using a same number of subcarriers as anumber of subcarriers equal to or smaller than a predetermined numberused for transmitting a connection request signal, the base stationapparatus including: subcarrier number detection means for detecting thenumber of subcarriers used for transmitting the connection requestsignal; a bandpass filter having a passband with a variable widthaccommodating the predetected number of subcarriers, for separating asignal of the mobile station apparatus falling within the passband froma received signal; and bandwidth control means for controlling apassband width of the bandpass filter in accordance with the number ofsubcarriers detected by the subcarrier number detection means.

According to the present invention, the base station apparatus controlsthe passband width of the bandpass filter to separate a signal (adesired signal) of the mobile station apparatus from a received signal,in accordance with the number of subcarriers used for transmitting aconnection request signal. For example, when the number of subcarriersused for transmitting a connection request signal is smaller than thenumber of subcarriers which can be accommodated by the passband width ofthe bandpass filter, the base station apparatus reduces the passbandwidth of the bandpass filter so as to accommodate the number ofsubcarriers used for transmitting the connection request signal.Therefore, the ratio of noise contained in a signal of the mobilestation apparatus, which passes the bandpass filter, is decreased andreception quality of the signal (for example, signal-to-noise ratio) isimproved. That is, it is possible to substantially extend acommunication area for the base station apparatus.

In an aspect of the present invention, the subcarrier number detectionmeans may detect the number of subcarriers used for transmitting theconnection request signal, based on a signal received together with theconnection request signal. In this aspect, the subcarrier numberdetection means may detect the number of subcarriers used fortransmitting the connection request signal, based on an intensity ofeach subcarrier component in a signal received together with theconnection request signal. According to this aspect, the base stationapparatus can obtain the number of subcarriers used for transmitting aconnection request signal without receiving from the mobile stationapparatus a separate notice about the number of subcarriers.

In another aspect of the present invention, the base station apparatusmay communicate with the mobile station apparatus according to anorthogonal frequency division multiplexing system. According to thisaspect, in a mobile communication system employing an orthogonalfrequency division multiplexing method, it is possible to extend acommunication area of the base station apparatus.

Further, the present invention provides a reception band control methodfor a base station apparatus for performing radio communication with amobile station apparatus for transmitting a signal after connection isestablished, by using a same number of subcarriers as a number ofsubcarriers equal to or smaller than a predetermined number used fortransmitting a connection request signal, the method including:detecting the number of subcarriers used for transmitting the connectionrequest signal; and controlling in accordance with the detected numberof subcarriers a passband width of a bandpass filter having a passbandwith a variable width accommodating the predetected number ofsubcarriers, for separating a signal of the mobile station apparatusfalling within the passband from a received signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A diagram illustrating an entire configuration of a mobilecommunication system according to an embodiment of the presentinvention.

FIG. 2 A sequence diagram illustrating link channel establishmentprocessing (at a time of an incoming call) according to the embodimentof the present invention.

FIG. 3 A functional block diagram of a mobile station according to theembodiment of the present invention.

FIG. 4 A functional block diagram of a base station according to theembodiment of the present invention.

FIG. 5A A diagram illustrating the number of subcarriers andtransmission power per subcarrier before change.

FIG. 5B A diagram illustrating the number of subcarriers and thetransmission power per subcarrier after change.

FIG. 6A A diagram illustrating a passband width of a finite impulseresponse (FIR) filter before change.

FIG. 6B A diagram illustrating the passband width of the FIR filterafter change.

FIG. 7 A flowchart illustrating the link channel establishmentprocessing (at the time of the incoming call), which is executed by themobile station, according to the embodiment of the present invention.

FIG. 8 A flowchart illustrating the link channel establishmentprocessing (at the time of the incoming call), which is executed by thebase station, according to the embodiment of the present invention.

FIG. 9 A diagram illustrating a system bandwidth and a subchannelbandwidth in a mobile communication system employing an OFDMA system.

BEST MODE FOR CARRYING OUT THE INVENTION

A detailed description is made below of an embodiment of the presentinvention with reference to the drawings.

FIG. 1 is a diagram illustrating an entire configuration of a mobilecommunication system 10 according to the embodiment of the presentinvention. As illustrated in FIG. 1, the mobile communication system 10includes a base station 12 and a plurality of mobile stations 14 (onlythree mobile stations are illustrated in FIG. 1).

The base station 12 performs multiplex communication with the pluralityof mobile stations 14 according to a time division multiple access/timedivision duplex (TDMA/TDD) system and an orthogonal frequency divisionmultiple access (OFDMA) system.

The mobile station 14 is, for example, a portable mobile phone, apersonal digital assistant, or a communication card. The mobile station14 has a transmission power upper limit value defined therefor, and isnot allowed to increase transmission power beyond the upper limit valueeven in a case of an insufficient uplink budget.

The mobile communication system 10 has a common channel (CCH) andindividual channels (ICH) defined therefor as radio channels forperforming radio communication between the base station 12 and theplurality of mobile stations 14. The CCH is used in common between thebase station 12 and the plurality of mobile stations 14. The ICHs areused between the base station 12 and the respective mobile stations 14.Each of the radio channels is constituted by one or a plurality ofsubchannels composed of a plurality of subcarriers orthogonal to eachother (see FIG. 9). In this regard, the maximum number of subcarriersconstituting one radio channel is limited. It should be noted that, ifone radio channel is constituted by two or more subchannels, the two ormore subchannels may be adjacent to or apart from each other. Further,the plurality of subcarriers constituting one subchannel may be adjacentto or apart from each other.

The CCH and the ICH have a plurality of function channels definedtherefor, which are used in the respective phases of communication. Forexample, for the CCH, a paging channel (PCH), a timing correct channel(TCCH), a signaling control channel (SCCH), and the like are defined.For the ICE-I, an individual control channel (ICCH), a traffic channel(TCH), and the like are defined.

FIG. 2 is a sequence diagram illustrating link channel establishmentprocessing at a time of an incoming call in the mobile communicationsystem 10. As illustrated in FIG. 2, at the time of the incoming call,the base station 12 concurrently transmits call signals notifying theincoming call to the mobile stations 14 that exist in its communicationarea on the PCH (S100). On the other hand, each of the mobile stations14, which has received the call signal addressed thereto, transmits aconnection request signal for requesting link channel establishment tothe base station 12 on the TCCH (S102).

At this time, if the uplink budget of the mobile station 14 is smallerthan a distance between the mobile station 14 and the base station 12,the connection request signal transmitted in S102 does not arrive at thebase station 12. In this case, the mobile station 14 decreases thenumber of subcarriers and increases transmission power per subcarrier(S104) to retransmit the connection request signal (S106). Thisfacilitates the arrival of the connection request signal transmittedfrom the mobile station 14 at the base station 12.

When the connection request signal retransmitted in S106 arrives at thebase station 12, the base station 12 detects the number of subcarriersof the TCCH, which have been used for transmitting the connectionrequest signal, and changes a reception bandwidth according to thenumber of the detected subcarriers (S108). In this case, the receptionbandwidth of the base station 12 in S106 is wide with respect to thenumber of the detected subcarriers of the TCCH, and hence the basestation 12 narrows the reception bandwidth to be applied to thesubsequent reception. After that, the base station 12 determines the ICHthat is to be allocated to the mobile station 14 and transmits aconnection response signal containing the determined ICH to the mobilestation 14 on the SCCH (S110). When the mobile station 14 receives theconnection response signal from the base station 12, the link channelestablishment between the base station 12 and the mobile station 14 iscompleted. Then, the mobile station 14 transmits, to the base station12, an allocation confirmation signal for confirming the allocation ofthe ICH by using the ICH (ICCH) (S112), to thereby start the radiocommunication using the ICH between the mobile station 14 and the basestation 12 (S114).

In the mobile communication system 10, transmission conditions (numberof the subcarriers and transmission power per subcarrier) according tothe connection request signal arriving at the base station 12 in S106are also applied to radio transmission performed by the mobile station14 in S112 and thereafter. Therefore, the uplink budget is improved.Further, the reception bandwidth of the base station 12, which isnarrowed in S108, is maintained thereafter, and hence the base station12 has improved reception quality of a signal. In this manner, themobile communication system 10 realizes extension of the communicationarea of the base station 12.

A detailed description is made below of configurations of the mobilestation 14 and the base station 12 for realizing the above-mentionedprocessing.

FIG. 3 is a functional block diagram of the mobile station 14. Asillustrated in FIG. 3, the mobile station 14 includes an antenna 20, atransmission/reception unit 22, an OFDM signal processing unit 24 (OFDMdemodulation section 26, subcarrier control section 28, and OFDMmodulation section 30), and a control unit 32 (connection requesttransmission section 34, connection response detection section 36,retransmission control section 38, transmission channel control section40, and transmission power control section 42).

The antenna 20 receives a radio signal and outputs the received radiosignal to the transmission/reception unit 22. Further, the antenna 20radiates, to the base station 12, a radio signal supplied from thetransmission/reception unit 22. Transmission and reception of the radiosignal are switched in a time-division manner based on an instructionissued by the transmission/reception unit 22.

The transmission/reception unit 22 includes a low noise amplifier, apower amplifier, a local oscillator, a mixer, and a filter. The radiosignal input from the antenna 20 is amplified by the low noiseamplifier, downconverted into an intermediate frequency signal, and thenoutput to the OFDM signal processing unit 24. Further, an intermediatefrequency signal input from the OFDM signal processing unit 24 isupconverted into a radio signal, amplified to a transmission power levelby the power amplifier, and then supplied to the antenna 20. It shouldbe noted that the power amplifier performs amplification so thattransmission power of each of the subcarriers contained in the radiosignal becomes transmission power per subcarrier notified from thetransmission power control section 42 described later.

The OFDM signal processing unit 24 functionally includes the OFDMdemodulation section 26, the subcarrier control section 28, and the OFDMmodulation section 30.

The OFDM demodulation section 26 includes an A/D converter, a fastFourier transform (FFT) portion, and a symbol demapper. The intermediatefrequency signal input from the transmission/reception unit 22 to theOFDM demodulation section 26 is converted into a digital signal by theA/D converter, and converted into subcarrier components of complexsymbol sequence by the FFT portion executing a Fourier transform. Thesubcarrier components of complex symbol sequence are converted into asymbol sequence through parallel-serial conversion, and decoded into adata bit sequence (received data) according to a modulation scheme ofsymbols by the symbol demapper before being output to the control unit32.

The subcarrier control section 28 controls the OFDM modulation section30 so that the number of subcarriers used for transmitting the radiosignal is equal to the number of subcarriers notified by thetransmission channel control section 40 described later.

The OFDM modulation section 30 includes a D/A converter, an inverse fastFourier transform (IFFT) portion, and a symbol mapper. A data bitsequence (transmission data) input from the control unit 32 to the OFDMmodulation section 30 is converted into a complex symbol sequence by thesymbol mapper, and then divided into subcarrier components throughserial-parallel conversion. This serial-parallel conversion processingis controlled by the subcarrier control section 28 so that the number ofsubcarriers of complex symbol sequence is equal to the number ofsubcarriers notified by the transmission channel control section 40. Thesubcarrier components of complex symbol sequence thus obtained areconverted into OFDM symbol sample values by inverse Fourier transformexecuted by the IFFT portion. The OFDM symbol sample values areconverted into an analog signal by the D/A converter, and then output tothe transmission/reception unit 22 as a baseband OFDM signal (modulationsignal). This baseband OFDM signal is constituted by subcarriers, thenumber of which is equal to the number of subcarriers notified by thetransmission channel control section 40.

It should be noted that the subcarrier control section 28, the FFTportion, the IFFT portion, the symbol mapper, and the symbol demapperare each constituted by, for example, a digital signal processor (DSP).

The control unit 32 is constituted by, for example, a CPU, a memory, andthe like, and has functions of controlling the components of the mobilestation 14 by the CPU executing programs stored in the memory. Inparticular, the control unit 32 functionally includes the connectionrequest transmission section 34, the connection response detectionsection 36, the retransmission control section 38, the transmissionchannel control section 40, and the transmission power control section42.

The connection request transmission section 34 transmits the connectionrequest signal to the base station 12 (outputs a data bit sequencecorresponding to the connection request signal to the OFDM modulationsection 30) when the mobile station 14 starts radio communication withthe base station 12. Further, the connection request transmissionsection 34 retransmits the connection request signal in response to aninstruction issued by the retransmission control section 38 describedlater.

The connection response detection section 36 detects whether or not theconnection response signal is received from the base station 12 within apredetermined period of time after the connection request transmissionsection 34 transmits the connection request signal (whether or not adata bit sequence corresponding to the connection response signal isinput from the OFDM demodulation section 26 to the control unit 32). Forexample, the connection response detection section 36 may start timercounting at a timing at which the connection request signal istransmitted, and judge whether or not the connection response signal isreceived before a timer count value reaches a predetermined value.

If the connection response signal is not received within thepredetermined period of time after the connection request signal istransmitted, the retransmission control section 38 instruct thetransmission channel control section 40 to decrease the number of thesubcarriers used for transmitting the radio signal to the base station12, and instructs the transmission power control section 42 to increasethe transmission power per subcarrier by an amount of powercorresponding to an amount of decrease in the number of the subcarriers.Further, the retransmission control section 38 instructs the connectionrequest transmission section 34 to retransmit the connection requestsignal. It should be noted that the retransmission control section 38may count the number of times the connection request transmissionsection 34 retransmits the connection request signal, and limitretransmission of the connection request signal when the number of timesthe retransmission is performed reaches a predetermined number of times.

The transmission channel control section 40 controls subchannels(transmission channel) used for transmitting the radio signal to thebase station 12, such as TCCHs or ICCHs. In this embodiment, the numberof subcarriers constituting a transmission channel is determined in alink channel establishment phase.

First, the transmission channel control section 40 sets the number ofsubcarriers of the TCCH used for transmitting the connection requestsignal to an initial number determined in the mobile communicationsystem 10 (see FIG. 5A). Then, if the link channel establishment iscompleted without retransmitting the connection request signal, theinitial number of subcarriers are used for subsequently transmitting theradio signals (transmission through the ICCH or the TCH or the like).

On the other hand, if the connection request signal is retransmitted inthe link channel establishment phase, the transmission channel controlsection 40 sets a number that is smaller by one or more than the initialnumber as the new number of subcarriers of the TCCH in response to aninstruction issued by the retransmission control section 38, andnotifies of the new number the subcarrier control section 28 and thetransmission power control section 42 (see FIG. 5B). Then, also in acase where the connection request signal is retransmitted twice or more,the transmission channel control section 40 sets a number that issmaller by one or more than the number of subcarriers used for previoustransmission of the connection request signal as the new number ofsubcarriers of the TCCH. In this way, the transmission channel controlsection 40 gradually decreases the number of subcarriers of the TCCHfrom the initial number according to the number of times the connectionrequest signal is transmitted until the connection response signal isreceived from the base station 12. If the connection response signal isreceived from the base station 12, the same number of subcarriers asthat at the timing of the reception are used for subsequentlytransmitting the radio signals.

The transmission power control section 42 controls the transmissionpower used when the radio signal is transmitted to the base station 12.In particular, when the number of subcarriers of the TCCH is decreasedin the link channel establishment phase, the transmission power controlsection 42 determines new transmission power so that the transmissionpower per subcarrier is increased by the amount of power correspondingto the amount of decrease in the number of the subcarriers (see FIG.5B), and notifies the transmission/reception unit 22 of the determinedtransmission power per subcarrier.

It should be noted that the transmission power control section 42 mayset, as new transmission power per subcarrier, a value obtained bydividing a transmission power upper limit value for the mobile station14 by the number of subcarriers notified by the transmission channelcontrol section 40. With this configuration, the transmission power persubcarrier may be maximized within a range that does not exceed thetransmission power upper limit value.

In this way, if the connection response signal is not received from thebase station 12 within the predetermined period of time after theconnection request signal is transmitted, the mobile station 14decreases the number of subcarriers constituting the transmissionchannel and increases the transmission power per subcarrier by theamount of power corresponding to the amount of decrease in number toretransmit the connection request signal. Therefore, a range of theradio signal transmitted from the mobile station 14 to the base station12 may be extended without increasing the power consumption of themobile station 14.

FIG. 4 is a functional block diagram of the base station 12. Asillustrated in FIG. 4, the base station 12 includes an antenna 50, atransmission/reception unit 52, an OFDM signal processing unit 54(finite impulse response (FIR) filter 56, OFDM demodulation section 58,subcarrier number detection section 60, FIR filter control section 62,OFDM modulation section 64), and the control unit 66.

The antenna 50 receives a radio signal and outputs the received radiosignal to the transmission/reception unit 52. Further, the antenna 50radiates, to the mobile station 14, a radio signal supplied from thetransmission/reception unit 52. Transmission and reception of the radiosignal are switched in a time-division manner based on an instructionissued by the transmission/reception unit 52.

The transmission/reception unit 52 includes a low noise amplifier, apower amplifier, a local oscillator, a mixer, and a filter. The radiosignal input from the antenna 50 is amplified by the low noiseamplifier, downconverted into an intermediate frequency signal, and thenoutput to the OFDM signal processing unit 54. Further, an intermediatefrequency signal input from the OFDM signal processing unit 54 isupconverted into a radio signal, amplified to a transmission power levelby the power amplifier, and then supplied to the antenna 50.

The OFDM signal processing unit 54 functionally includes the FIR filter56, the OFDM demodulation section 58, the subcarrier number detectionsection 60, the FIR filter control section 62, and the OFDM modulationsection 64.

The FIR filter 56 is a bandpass filter having a passband with a variablewidth which accommodates an upper limit number of subcarriersconstituting a subchannel. The FIR filter 56 outputs, of theintermediate frequency signals input from the transmission/receptionunit 52, only signals falling within the passband to the OFDMdemodulation section 58. The passband of the FIR filter 56 is controlledby the FIR filter control section 62 described later so that a signal ofthe mobile station 14 is separated from a received signal. However, thepassband width of the FIR filter 56 is maintained to a bandwidth(hereinafter, referred to as “initial bandwidth”) corresponding to theinitial number of subcarriers until the connection request signal isreceived from the mobile station 14.

The OFDM demodulation section 58 includes an A/D converter, an FFTportion, and a symbol demapper. The intermediate frequency signal inputfrom the FIR filter 56 is converted into a digital signal by the A/Dconverter, and converted into subcarrier components of complex symbolsequence by the FFT portion executing a Fourier transform. Thesubcarrier components of complex symbol sequence are converted into asymbol sequence through parallel-serial conversion, and decoded into adata bit sequence (received data) according to a modulation scheme ofsymbols by the symbol demapper before being output to the control unit66.

The subcarrier number detection section 60 detects, based on thereceived signal, the number of the subcarriers of the TCCH used fortransmitting the connection request signal. For example, the number ofsubcarrier components that are included in the subcarrier components ofcomplex symbol sequence of a first radio signal (connection requestsignal), which are obtained by the OFDM demodulation section 58 when theconnection request signal is received from the mobile station 14 on theTCCH, and have signal intensities equal to or larger than apredetermined value may be detected as the number of subcarriers of theTCCH.

The FIR filter control section 62 changes a voltage applied to a controlterminal of the FIR filter 56, to thereby control the passband width ofthe FIR filter 56. As described above, the FIR filter control section 62maintains the passband width of the FIR filter 56 to the initialbandwidth until the connection request signal is received from themobile station 14 (see FIG. 6A).

When the connection request signal is received from the mobile station14 on the TCCH, the FIR filter control section 62 changes the passbandwidth of the FIR filter 56 according to the number of subcarriersdetected by the subcarrier number detection section 60. For example, ifthe received connection request signal is transmitted by using theinitial number of subcarriers illustrated in FIG. 5A, the FIR filtercontrol section 62 maintains the passband width of the FIR filter 56without change (see FIG. 6A). On the other hand, if the receivedconnection request signal is transmitted by using the subcarriersillustrated in FIG. 5B, the FIR filter control section 62 narrows thepassband width of the FIR filter 56 to a passband width illustrated inFIG. 6B, and applies this passband width to signals subsequentlyreceived from the mobile station 14. With this configuration, a ratio ofnoise contained in a signal of the mobile station 14, which passes theFIR filter 56, is decreased and reception quality of the signal (forexample, signal-to-noise ratio) is improved.

Next, a description is given of operations of the mobile station 14 andthe base station 12.

FIG. 7 is a flowchart illustrating the link channel establishmentprocessing at the time of the incoming call, which is executed by themobile station 14. As illustrated in FIG. 7, when the mobile station 14receives the call signal addressed thereto from the base station 12 onthe PCH (S200), the mobile station 14 transmits the connection requestsignal to the base station 12 through the TCCH (S202). After that, themobile station 14 monitors whether or not the connection response signalis received from the base station 12 on the SCCH (S204). If the mobilestation 14 receives the connection response signal within thepredetermined period of time after the mobile station 14 transmits theconnection request signal (S204: Y), the link channel establishment withrespect to the base station 12 is completed.

In this case, the mobile station 14 transmits the allocationconfirmation signal to the base station 12 by using the ICH (ICCH)notified in the connection response signal (S206), to thereby startcommunication with the base station 12 (S208).

On the other hand, if the mobile station 14 does not receive theconnection response signal within the predetermined period of time afterthe mobile station 14 transmits the connection request signal in S202,except for a case where the number of times the connection requestsignal is retransmitted reaches the predetermined number of times (S210:Y), the mobile station 14 decreases the number of the subcarriers of thetransmission channel used for the TCCH and the subsequent communicationby one or more (S212), and increases the transmission power persubcarrier by the amount of power corresponding to the amount ofdecrease in the number of the subcarriers (S214). Then, the mobilestation 14 retransmits the connection request signal to the base station12 through the TCCH (S216) to execute the processing of S204 and thesubsequent processing.

FIG. 8 is a flowchart illustrating the link channel establishmentprocessing at the time of the incoming call, which is executed by thebase station 12. As illustrated in FIG. 8, first, the base station 12concurrently transmits call signals to the mobile stations 14 that existin its communication area on the PCH (S300). After receiving theconnection request signal from the mobile station 14 corresponding tothe call signal through the TCCH (S302), the base station 12 detects thenumber of subcarriers constituting the TCCH, and judges whether or notthe number of subcarriers is equal to the initial number determined inthe mobile communication system 10 (S304).

Here, if the detected number of subcarriers is equal to the initialnumber (S304: Y), the base station 12 uses the SCCH to transmit theconnection response signal containing the ICH to the mobile station 14without changing the passband width of the FIR filter 56 (S308). On theother hand, if the number of subcarriers detected in S304 is smallerthan the initial number (S304: N), the base station 12 narrows thepassband width of the FIR filter 56 according to the detected number ofsubcarriers (S306), and then transmits the connection response signalcontaining the ICH (S308). After that, when the base station 12 receivesthe allocation confirmation signal from the mobile station 14 on the ICH(ICCH) (S310), the base station 12 starts the communication with themobile station 14 (S312).

According to the mobile communication system 10 described above, therange (uplink budget) of the radio signal transmitted from the mobilestation 14 to the base station 12 is improved, and the communicationarea of the base station 12 may be extended. Further, the base station12 has the improved reception quality of a signal, and hence thecommunication area of the base station 12 may be further extended.

It should be noted that the present invention is not limited to theabove-mentioned embodiment. For example, the present invention is notlimited to a mobile communication system employing both a TDMA/TDDsystem and an OFDMA system, and may be widely applied to communicationsystems employing a multicarrier communication system.

Further, although not particularly referred to in the above descriptionof the embodiment, the number of subcarriers in a downlink direction(direction from the base station 12 to the mobile station 14) may beconstant or variable.

1. A base station apparatus for performing radio communication, with amobile station apparatus for transmitting a signal after connection isestablished, by using a same number of subcarriers as a number ofsubcarriers equal to or smaller than a predetermined number used fortransmitting a connection request signal, the base station apparatuscomprising: subcarrier number detection means for detecting the numberof subcarriers used for transmitting the connection request signal; abandpass filter having a passband with a variable width accommodatingthe predetected number of subcarriers, for separating a signal of themobile station apparatus falling within the passband from a receivedsignal; and bandwidth control means for controlling a passband width ofthe bandpass filter in processing the received signal after theconnection is established, in accordance with the number of subcarriersdetected by the subcarrier number detection means.
 2. The base stationapparatus according to claim 1, wherein the subcarrier number detectionmeans detects the number of subcarriers used for transmitting theconnection request signal, based on a signal received together with theconnection request signal.
 3. The base station apparatus according toclaim 2, wherein the subcarrier number detection means detects thenumber of subcarriers used for transmitting the connection requestsignal, based on an intensity of each subcarrier component in a signalreceived together with the connection request signal.
 4. The basestation apparatus according to claim 3, wherein the base stationapparatus communicates with the mobile station apparatus according to anorthogonal frequency division multiplexing system.
 5. The base stationapparatus according to claim 1, wherein the base station apparatuscommunicates with the mobile station apparatus according to anorthogonal frequency division multiplexing system.
 6. The base stationapparatus according to claim 2, wherein the base station apparatuscommunicates with the mobile station apparatus according to anorthogonal frequency division multiplexing system.
 7. A reception bandcontrol method for a base station apparatus for performing radiocommunication with a mobile station apparatus for transmitting a signalafter connection is established, by using a same number of subcarriersas a number of subcarriers equal to or smaller than a predeterminednumber used for transmitting a connection request signal, the methodcomprising: detecting the number of subcarriers used for transmittingthe connection request signal; and controlling in accordance with thedetected number of subcarriers a passband width of a bandpass filterhaving a passband with a variable width accommodating the predetectednumber of subcarriers, for separating a signal of the mobile stationapparatus falling within the passband from a received signal, whereinthe passband width is controlled during processing of the receivedsignal after the connection is established.